Optical finger navigation (OFN) modules are becoming increasingly common in data processing systems, such as cellular telephones, tablet computers, electronic readers and portable entertainment or game systems for data input and/or cursor movement. OFN modules include optical navigation sensors (ONS), which generally include a light source to illuminate a tracking surface, such as a finger or stylus in contact with a surface of the OFN module, and a sensor, such as a charge-coupled device (CCD), complementary metal-oxide-semiconductor (CMOS) imaging array, or a photo-diode array, to capture an image or signal in light reflected from the surface. A tracking program implemented in a signal processor coupled to the sensor analyzes successive images or signals to determine displacement of the optical navigation module relative to the surface.
One problem with existing OFN modules is that the ONS cannot differentiate between light reflected from a finger or stylus and environmental or ambient light. Strong ambient light can cause spurious cursor motions known as auto-movements. Sunlight is a particular problem, and manufacturers usually requires that the OFN module pass strict sunlight tests with specified range of light intensity conditions, test time, test angle, orientations relative to the sunlight. Prior approaches to addressing the problem of auto-movement focused on mechanical solutions in an attempt to block the sunlight. This has not been convenient, cost effective, or entirely effective since sunlight is inevitably transmitted through a window of the OFN module.